The use of PEMS as a base upon which to fabricate electronic circuitry is well known in the art. The early PEMS comprised a conventional porcelain enamel glass applied to a metal substrate, typically a low carbon steel substrate, which was fired causing the glass to fuse and flow and form a glass film bonded to the metal substrate by various chemical and mechanical mechanisms. On cooling to room temperature, the resulting porcelain enameled metal substrate was found to be useful for fabricating a variety of electronic elements, so long as the subsequent steps in the fabrication of the electronic component did not require reheating of the PEMS to a temperature higher than the softening temperature of the glass, usually between 550.degree. and 650.degree. C. This temperature limitation prohibited the use of PEMS in the fabrication of high reliability hybrid circuit boards where the printed circuits were desired to be cured at temperatures as high as 850.degree. C.
In preparing such PEMS from porcelain enamel glass compositions, the glass was typically prepared from a mixture of precursor oxides which were smelted at a temperature of about 1200.degree. to about 1500.degree. C. for about 30 to about 60 minutes, then roll quenched to provide a frit which was ground in water to an average particle size of about 10 to about 25 microns. The porcelain enamel overcoat was then applied to the metal substrate by a number of different techniques, but usually by electrophoresis from a water based bath, after which the enamel was dried and fired typically at a temperature of about 760.degree. to about 880.degree. C.
As noted earlier, in order to achieve stability and reliability of printed hybrid circuit boards, refiring temperatures as high as 850.degree. C. or higher are preferred. Since conventional porcelain enamel compositions could not function under such parameters, the use of crystallizing glass compositions were developed. Crystallizing glass compositions, or devitrified glass compositions, behave quite differently from conventional porcelain enamel glass compositions when fired.
The conventional glass enamel, when applied to the metal substrates and fired, are fused with increasing temperature to flow and form a glass film upon the metal substrate. The crystallizing porcelain enamel coatings on the other hand have a normal firing temperature of about 800.degree. to 900.degree. C. to form a good enamel film, but subsequently crystallize to drastically increase the viscosity of the coating. The crystallized coating will then behave similar to the crystalline materials and retain their rigidity even if refired to the same high firing temperatures. Such materials, for example, are taught in U.S. Pat. No. 4,256,796 to Hang et al., U.S. Pat. No. 4,358,541 to Andrus et al., U.S. Pat. No. 4,385,127 to Chyung, and also in Japanese Patent Publication No. 85/172 102, dated Sept. 5, 1985. These glass ceramic systems generally rely on the recrystallization of BaO.2MO.2SiO.sub.2, 2MO.B.sub.2 O.sub.3, 2MO.SiO.sub.2, and MO.SiO.sub.2 crystals from the mother glass based on the various oxide compositions of BaO, SiO.sub.2, B.sub.2 O.sub.3, and MO, where MO stands for MgO, CaO, or ZnO.
The use of recrystallizing glass ceramic coatings has overcome the softening problems encountered with conventional glass enamel compositions, however, problems have been encountered with lack of adhesion of the glass ceramic coating to the metal substrate. In particular with low carbon steel substrates, it has been noted that the enamel cracks on the edges of the substrate, more particularly on the corners, especially after being refired repeatedly at high temperatures. This failure is due in part to the poor adhesion of glass ceramic coating to the metal. However, the main cause of the enamel cracks is due to too high a thermal expansion of the core metal compared to that of the coating material. The volume change accompanied by the ferrite-toaustenite phase transformation in low carbon steel taking place around 870.degree. C., is still another factor causing the enamel cracks if fired above this temperature.
The low carbon steel substrates were also found to have a tendency to warp when fired repeatedly at temperatures in excess of 850.degree. C. Extended exposure to elevated temperatures can cause the grain growth of low carbon steel structures and the coarse grain crystal structures adversely affect the physical strength of substrates. The volume change associated with the phase transformation can further distort the low carbon steel substrate.
Nevertheless, at present, the most popular substrate for enamel coating is still low carbon steel. Typically where low carbon steels are employed as the metal substrate, they are subjected to acid pickling and deposition of a thin nickel coating prior to the electrophoretic application of the porcelain enamel coating. Such techniques, however, have limited efficacy in the case of alloy metal substrates such as stainless steel which are chemically and electrochemically rather inert, so that little or no reaction takes place in the acids and/or the nickel sulfate solution. Until now, where stainless steel was to be employed as the metal substrate for fabrication of a PEMS the stainless steel was either etched and/or sandblasted. While sandblasting is generally considered the state of the art method, it is expensive and often deforms or warps the light gauge sheet steels commonly used as substrates for enameling. In addition, sand particles can become imbedded in the steel surface and cause enamel defects.
U.S. Pat. No. 3,962,490 to Ward teaches a method of applying an enamel coating to a stainless steel substrate in which the stainless steel workpiece is first dipped into an aqueous solution of a molybdenum salt, then heated to thermally decompose the molybdenum compound prior to enameling. This worked fine for some applications, but it did not provide an even coating of molybdenum, and was not suitable for PEMS being prepared for use in fabricating sophisticated electronic components.
The application of porcelain enamel coating on to the metal substrates can be done in various ways, however, electrophoretic deposition technique is most preferred for the manufacture of PEMS. Early PEMS were prepared by electrophoretically depositing the enamel from a water slurry. The chemistry of crystallizing glass ceramic coatings, however, generally requires a non-aqueous suspension such as alcohols.
The preparation of a suspension with the optimum properties may require experimentation with the composition, concentration, and dispersing procedure. Polar compounds such as alcohols, nitroparaffins, and mixtures of these can be employed. Slightly polar organic compounds, such as diethylene glycol, dimethyl ether, and pyridine may also be employed. A great number of suspension formulations can be made from these various organic suspension vehicles, but no single generalized formulation can be defined.
It is therefore one object of the present invention to provide a novel crystallizing porcelain enamel composition for use in coating metal substrates.
It is another object of the present invention to provide a novel deposition bath for electrophoretically applying crystallizing porcelain enamel coatings to a metal substrate.
It is yet a further object of the present invention to provide improved porcelain enameled metal substrates for use in fabrication of electronic circuitry.
It is a still further object of the invention to provide a novel method of treating steel substrates to enhance the adhesion of the fired porcelain enamel coating to the metal substrates, specifically electrochemically rather inert alloy steels such as stainless steels.